1. Field of the Invention
The present invention relates to a temperature measuring device utilizing a thermocouple, in particular, to a temperature measuring device which makes it possible to greatly reduce lengths of thermocouple extension lead wires thereof or even to eliminate the use of such wires altogether.
2. Description of the Prior Art
In a prior art temperature measuring device with a thermocouple, pieces of two kinds of metal are joined together at two points one of which is used as the measuring point while the other is used as the reference point. Metals thus joined are cut across at an appropriate point, and the temperature determination is made based on the potential difference created between the cut ends at the appropriate point. In the above device, the reference point is placed in a thermostatic vessel such as an ice-filled water tank in order to maintain its temperature at a constant value.
However, in this system, the price for the device becomes high due to the use of the large size of the thermostatic vessel for the reference point. Moreover, it requires much labor to operate the device because of the maintenance requirements, such as the monitoring of temperature of the thermostatic vessel. As a result, that device is very poorly suited for various industrial purposes.
As other devices, there is known a device which can dispense with the thermostatic vessel.
This device has a reference contact compensator which includes a constant voltage source, resistors, and a current source. The reference contact compensator detects the variation in the temperature at the reference contact and compensates the corresponding variations in the output voltage, in order to ensure a constant temperature at the reference contact.
However, in a reference contact compensator like the one in the above, the length of the thermocouple extension lead wires which connect the thermocouple to the reference contact is necessarily large. This is because the reference contact is provided either closer to the processing circuit for the output voltage than the measuring point or within the processing circuit, due to the reasons for heat-proofing and installing convenience. Although the lead wires are less expensive than the thermocouple, they are still more expensive than the ordinary lead wires, due to the requirement of a pair of lead wires with similar thermoelectromotive forces, resulting in a high cost for the device. Furthermore, the thermocouple extension lead wires, due to their compensating function in accordance with the variations in the temperature, have different compositions of the constituent metals, and hence naturally different electrical resistances. Therefore, the difference in resistances increases in proportion to the length of the lead wires. Then, in the view from the processing circuit, the impedances at the input terminals of the processing circuit are out of balance. Therefore, it is difficult to eliminate the external noise component which is mixed into the transmission route and the common mode voltage element which arises from the insulation resistance of the thermocouple relative to the ground. This causes an increase in the errors of the measurements. Moreover, an extremely high accuracy is required for the reference contact compensator due to the fact that it deals with the compensation voltage which is smaller than the thermoelectromotive forces that is on the order of milli-volt. As a result, the selection of the elements themselves and the adjustment of the circuit become extremely time-consuming. However, for the troubles taken, the resulting accuracy and stability of the device are often not up to the expectation. In short, the prior art devices in the above suffer from the problems that are described in the foregoing.